The present invention relates to methods and apparatuses for non destructive internal inspection of pieces by ultrasound, by comparing the response from a piece to be inspected with a reference response constituted either by the response from a piece that is taken to be satisfactory, or from an average value of responses from a plurality of pieces that are unlikely to present the same defect, or else from a mathematical model.
A particular important, but not exclusive, application of the invention lies in detecting faults, breaks, cracks, and non-uniformities in various materials such as metals, composites, and ceramics.
Numerous apparatuses for ultrasound inspection of pieces are already known that operate in transmission or in reflection. When an in-depth image is to be made of a piece by using an array of transducers as a source and as a receiver, a major difficulty arises from the fact that the echo reflected from the input interface into the piece is much more intense than any echo which may be reflected from defects and the said interface echo masks the echoes to be identified. The problem is further aggravated when the piece is complex in shape and/or non-uniform in structure.
Since it is not possible to eliminate the interfering echo from the input interface (and possibly also echoes that come from multiple reflections on the input and output interfaces), the invention seeks to compensate for it.
To do this, the invention uses a technique that may be called "time-reversed or time inverted ultrasonic amplification" as described in document EP-A-0 383 650 to which reference may be made. That technique is itself based on the identity of waveform equations when time is reversed, even when dealing with signals over a very wide frequency band. According to document EP-A-0 383 650, a zone to be inspected is initially "illuminated" from one or more transducers belonging to a two-dimensional array, and the echoes from the material are recorded in electronic memories located behind each individual transducer. In a second stage, the received signals are re-emitted after inverting their time distributions, and possibly also their waveforms. In other words, the signals received last are returned first.
The effect of variation in absorption as a function of depth can be compensated by changing the gain with which the time-reversed or time inverted wave is amplified as a function of the to and fro time.
The purpose of such time reversal was to return a wave in the event of a wave representing an echo from a defect to the defect with an increased amplitude. The divergent wave front returned by a defect of arbitrary shape was thus optimally refocussed.